Back facet flared ridge for pump laser

ABSTRACT

A laser pump module has a ridge waveguide chip with a ridge width that is flared in the direction of the rear facet. Such a ridge profile yields a number of advantages depending on the specific implementation. The resulting increase in area of the rear facet, spreads heat, to improve device performance and avoid catastrophic optical mirror damage (COD). It can also lower the power density for the same output intensity. Further, it contributes to the single lateral mode property, acting as a mode filter. Higher order modes suffer higher losses and thus, tending to contribute to their attenuation—with less light fed back into the cavity from the rear facet, the laser device as a whole has reduced outside feedback sensitivity. Thus, in the implementation as a pump laser, less light at the signal wavelength returns to be amplified in a majority of the pump&#39;s ridge waveguide cavity. Still further, any back facet power monitor can be moved to improve its sensitivity level due to the large output and input area.

BACKGROUND OF THE INVENTION

Semiconductor laser devices such as ridge waveguide lasers and laseramplifiers are used in many communications systems. Incrementalrefinements in their fabrication and packaging have resulted in a classof devices that have acceptable performance characteristics and awell-understood long-term behavior. Moreover, weakly guiding ridgewaveguide structures are less complex to fabricate and provide excellentyields as compared to more complex architectures based on buriedheterostructures, for example.

In most applications, maximizing the laser's or amplifier's usefuloperating power is a primary design criteria. In signal laserapplications, the power output from the device dictates the distance tothe next repeater stage, and the number of stages in a given link is amajor cost factor in the link's initial cost and subsequent maintenance.In pump laser applications, where typically multiple pump laser devicesare used to optically pump a gain or amplifying fiber, such as arare-earth doped fiber amplifier or regular fiber in a Raman pumpingscheme, useful power output dictates the number of pumps required toreach a required pumping level and/or the distance between pump/fiberamplification stages.

Under current technology, the typical application for pump lasers isfiber amplification systems that utilize rare-earth doped fiber as thegain fiber. These gain fibers are located at attenuation-dictateddistances along the fiber link. They typically are comprised oferbium-doped fiber amplifiers (EDFA). The laser pumps typically operateat 980 nanometers (nm) or 1480 nm, which correspond to the location ofabsorption peaks for the EDFA's in the optical spectrum.

More recently, Raman pumping schemes have been proposed. The advantageis that special, periodic, EDFA amplifier gain fiber is not required tobe spliced into the fiber link. Instead, regular fiber can be used. Theresult is a gain spectrum that is even wider than systems relying onEDFA's. The bandwidth typically extends over the entire transmissionbandwidth for fiber, stretching from 1250 to 1650 nm for some fibercompositions. The pump lasers are designed to operate in the wavelengthrange of 1060 to 1500 nm in the typical implementation.

Advantages associated with Raman amplification systems surrounds thefact that there is no longer a 3dB noise penalty associated with eachamplifier, as occurs with EDFA's. Raman amplification, however, is anon-linear process. As a result, relatively high pump powers arerequired.

In any case, high power pumps are required, regardless of whether EDFA'sor regular/Raman systems are used. Currently, pumps yielding 180 to 200milliWatts (mW) of power are available. Newer system designs arerequiring even higher power pumps, however.

SUMMARY OF THE INVENTION

As higher pump powers are required, additional optimizations are pursuedin the pump laser module. One subject for these optimizations concernsthe laser pump chip within the module. Specifically, in the presentinvention, the ridge profile of the laser chip is optimized both forhigh output power and also operation as a pump device, in whichsignal-band light, at other then the pump wavelength, is present.

Specifically, the present invention concerns a ridge waveguide lasermodule in which the ridge is flared in the direction of the rear orreflecting facet. Such a ridge profile yields a number of advantagesdepending on the specific implementation. The resulting increase in areaof the rear facet spreads heat, to improve device performance and avoidcatastrophic optical mirror damage (COD). It also lowers the powerdensity for the same output intensity. Further, it contributes to thesingle lateral mode property, acting as a mode filter. Higher ordermodes suffer higher losses, which thus tends undermine the establishmentof those modes. Further, with less light fed-back into the cavity fromthe rear facet, the laser device as a whole has reduced outside feedbacksensitivity. Thus, in the implementation as a pump laser, less light atthe signal wavelength returns to be amplified in a majority of thepump's ridge waveguide cavity. Still further, any back facet powermonitor can be moved to improve its sensitivity level due to the largeoutput and input area.

In general, according to one aspect, the present invention concerns aridge waveguide pump laser module. The module is adapted to generatelight in the 1.2 to 1.6 micrometer wavelength range. It comprises aridge waveguide laser chip that has a back, reflective facet and a frontfacet. The pump light is emitted through this front facet. According tothe invention, the ridge width increases in the direction of the rearfacet.

In specific implementations, a medial to front section of the ridge hasa width of 1.5 to 7 micrometers. The rearward section of the ridge has awidest width of 3-14 micrometers. Specifically, in a preferredembodiment, the rearward section of the ridge has a maximum width of 8-9micrometers and a medial to front section of the ridge has a width of 5to 6 micrometers.

According to other specifics of the embodiments, the end of the opticalfiber pigtail, which is positioned to receive light generated by theridge waveguide laser chip, preferably comprises a flat-top wedge shapedfiber lens. The core of the fiber may also be flared in the direction ofthe chip and/or elliptical. Polarization maintaining fiber pigtails,such as polarization controlling fiber pigtails, are preferredespecially if a grating is included for power stabilization.

In the anticipated implementation, the module is used to pump an EDFAamplifier. Alternatively, non-rare-earth doped gain fiber is used as thegain fiber in a Raman amplification scheme, however.

The above and other features of the invention including various noveldetails of construction and combinations of parts, and other advantages,will now be more particularly described with reference to theaccompanying drawings and pointed out in the claims. It will beunderstood that the particular method and device embodying the inventionare shown by way of illustration and not as a limitation of theinvention. The principles and features of this invention may be employedin various and numerous embodiments without departing from the scope ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings, reference characters refer to the sameparts throughout the different views. The drawings are not necessarilyto scale; emphasis has instead been placed upon illustrating theprinciples of the invention. Of the drawings:

FIG. 1 is a perspective view of a pump laser module according to thepresent invention;

FIG. 2 is a schematic, perspective view of the inventive laser chip andcoupling techniques for the light generated by the chip into the fiberpigtail;

FIG. 3 is a top-plan view showing the ridge profile of the presentinvention;

FIGS. 4A-4D are cross-sectional views illustrating the process by whichthe inventive wide-ridge pump laser chip is manufactured;

FIGS. 5A and 5B are a cross-sectional views showing alternative ridgedesigns; and

FIGS. 6A-6B are top plan views illustrating different ridge profiles forthe present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a pump laser module, which has been constructed accordingto the principles of the present invention and which contains a ridgewaveguide laser chip according to the present invention.

Specifically, the ridge waveguide laser chip 110 is installed on asubmount 112 in the typical implementation. The submount 112 providesmechanical and electrical connections between the laser chip 110 and themodule housing 114. A fiber pigtail 118 extends through a side wall 120of the module housing 114. It is typically rigidly secured to thesubmount 112 such that the fiber endface 122 is held in proximity to anoutput facet 116 of the laser diode 110.

Depending on the implementation and application requirements, fiberpigtail 118 is constructed from regular or, alternatively,polarization-maintaining fiber. If used, different types ofpolarization-maintaining fiber are applicable. For example, panda,elliptical stress bar, and bow tie are viable substitutes as arepolarization controlling fiber.

In one implementation, a grating 128 is written into the regular fiber124 to create an external cavity to stabilize the operation of the chip110. These gratings are typically manufactured by UV beam interference.The gratings are usually written to the depth of the core in the fiber.In pump applications, they have the effect of stabilizing the moduleagainst temporal power output fluctuations.

In the preferred implementation, the fiber grating provides reflectivityat about 1450 nm, which is within the gain band of the diode laser 110.Further, the fiber grating has a bandwidth of 0.5-5 nm, preferably,although a range as large as 0.2-7 nm is required for someimplementations.

The polarization control maximizes and stabilizes the effect of thefiber grating. Light exiting from the typical diode laser is polarized.As a result, any light that is coupled into another, non-favored fiberaxis, i.e., the axis of the fiber that is not aligned with thepolarization of the laser diode if such axis exists, is reflected by thegrating, but has no effect on the laser diode since the diode isnonresponsive to light of this polarization.

If fiber pigtail 118 comprises PM fiber, it is preferably is opticallycoupled to a strand of regular fiber 124 via splice 126. Regular fiberpreferably has a standard circular cross section core, i.e., has nostress bar, or no fast or slow axes.

Although the coupling between the polarization-maintaining fiber pigtail118 and regular, non-polarization-maintaining fiber 24 is shown as adirect fusion splice. The critical feature is the optical couplingbetween the two fibers. Thus, other techniques for obtaining thiscoupling can be used such as intervening fiber lengths of a third fiber.

In a similar vein, the PM fiber need not directly receive the light fromthe diode. Instead, the light can be first coupled into a relativelyshort length of regular fiber for example, and then into PM fiber, whichtransmits the light over most of the distance to the grating. This isnot preferred, however, because of the need for additional splicing.

In any event, light coupled into the pigtail 118 is transmitted towardthe fiber amplification system. This system comprises an EDFA, in whichthe fiber is erbium-doped. Alternatively, as discussed previously, Ramanamplification principles can be used. In this case, the fiberamplification system 130 comprises regular fiber, or fiber which isoptimized for Raman amplification, and has the associated wide gainspectrum.

This module further comprises a thermo-electric cooler to provide forheat dissipation. However, for undersea applications, a coolerlessmodule is typically acceptable.

FIG. 2 is a schematic view showing the laser chip 110 and techniques forcoupling the light generated by the chip into the fiber pigtail 118.

Specifically, light is generated in the active layer 210 byrecombination and confined by a lower cladding layer 212 and an uppercladding layer 214. It is guided longitudinally by the ridge structure216, which has been etched into the top cladding layer 214. As a result,the light is confined to oscillate between a partially or antireflectingfront facet 116 and a highly reflecting rear facet 217. Most of thegenerated light exits from the laser chip 110 at a emission region 218,which generally is elliptically shaped.

Cladding layers of AlGaInAs or InGaAsP on an InP substrate arecompatible with the 1060-, 1200-1600 nm light generation. Quantum wellsof InGaAsP, AlGaInAs, or InGaAs are used, typically.

The emitted light forms a cone with an elliptical cross-section. The endof the fiber pigtail 11 8 is located such that coupling efficiency ismaximized. Most of the emitted light is captured to be transmitted bythe fiber pigtail 118.

One approach for maximizing coupling efficiency is to form a lens on theend of the fiber pigtail 118. Various fiber lens systems can be used,such as wedge-shaped fiber lenses, double-wedge-shaped fiber lenses,elliptical-cone-shaped fiber lenses, and lens systems includingelliptically-shaped lenses or cylindrical lenses, generally.

Presently, double-angle flat top microlenses are used as described inthe application entitled “Flat Top, Double-Angled, Wedge-Shaped FiberEndface”, U.S. Ser. No. 08/965,798, by Jeffrey Korn, Steven D. Conover,Wayne F. Sharfin and Thomas C. Yang, which is incorporated herein bythis reference.

The advantage of double-angle flat top microlenses is the fact that theyare not circularly symmetric with respect to the fiber's axis, tothereby match the elliptical spatial distribution of light from theoutput facet of the laser diode.

When non-circularly symmetric coupling techniques are used withpolarization-maintaining fiber, it is important to align the formedmicrolense, for example, with one of either the fast or slow axes of thepolarization-maintaining fiber, if two transmission axes are present.

Alternatively, a discrete lens or discrete lense system can also be usedas illustrated in FIG. 2. Specifically, a cylindrical lens 220 islocated between the end of the fiber pigtail and the emission region 218of the laser chip 110. Such a lens are typically used in combinationwith a simple cleaved-end surface of the fiber pigtail 118.Additionally, a fiber lens is used in combination with discrete lens 122in some embodiments, as illustrated.

Preferably, because of the elliptical emission of this wide ridge laser,a cylindrical lens as illustrated is preferably used. Alternatively, across-cylindrical discrete lens can also be implemented.

Alternately, the core 119 of the pigtail 118 has an ellipticalcross-section at least at an end which is positioned to receive lightgenerated by the laser chip 110. This configuration provides goodmatching to the elliptical emission 218 from the chip. Additionally, awidth of the core is tapered in a direction of light propagation awayfrom the laser chip, i.e., flared in the direction of the chip, tomaximize coupling efficiency, in some embodiments.

A P-metal contact layer 122 is typically located above the uppercladding layer 114 but separated by at least one insulation layer, notshown. This insulation layer is typically not present, however, betweenthe P-metal layer 222 and the upper cladding layer 214 in the region ofthe ridge 216. This allows a ridge injection current to be transmitteddown through the ridge 216 into the active layer 210. The ridgeinjection current is usually provided to the chip via a wire 224, whichis bonded to a bond pad 226 that is usually comprised of gold or goldalloy. The P-metal contact layer 222 is typically a gold alloy.

FIG. 3 is a top plan view showing the ridge profile according to thepresent invention. Generally, a medial to front portion of the ridge (A)has a width W. This width is preferably 1.5 to 7 micrometers. Currently,5-6 micrometers are believed to be optimum.

A rearward section of the ridge (B) is flared in the direction of therear facet 217. Preferably, the widest section of the rearward section(X) has a width of 3-14 micrometers. Preferably, the width is 8-9micrometers according to a present preferred embodiment, especially,when the front ridge width is 5 to 6 micrometers.

The overall length of the device (A+B)is usually between 500 and 2500μm, with about 1200 μm being preferred. The length of the rearwardsection is 10 to 200 μm preferably

As a result, the angle α is greater than 0 degrees in the presentinvention. In some implementations, it can be as great as 30°, however.In the typical implementation, the angle α is between 0.01° and 8°.

FIGS. 4A-4D illustrate the process by which the inventive flared-ridgepump laser chip is manufactured.

Specifically, as illustrated in FIG. 4A, the process starts with asemiconductor wafer substrate 310. This substrate has theepitaxially-grown lower cladding layer 212, active layer 210, and uppercladding layer 214.

An etchstop layer 312 is preferably provided in the upper cladding layerto control the etch depth of the subsequent ridge etch steps. In thepreferred embodiment, the distance between the active layer 210 and theetchstop layer 312 is relatively large, between 0.3 and 0.9 micrometers,possibly to as large as 1.1 micrometers in some embodiments. Thisdistance is consistent with the relatively wide ridge approach.Specifically, it yields a more weakly-guided chip, which guaranteessingle lateral mode operation to very high output power and alsominimizes the power loss by the lowest order mode. Specifically, in thepreferred embodiment, the distance between the active layer 210 and theetchstop layer 312 is between 0.5 and 0.7 μm.

On top of the upper cladding layer, insulation layers 314 are grown.These provide the electrical insulation between the upper cladding layer214 and the subsequent P-metal contact layers.

FIG. 4B shows the next step in which trenches 316, 318 are etched intothe upper cladding layer 214, to the depth of the etchstop layer 312 tothereby define the ridge 216. Subsequently, an additional insulationlayer 320 is formed over the substrate.

Next, photoresist layers 322 are deposited over the substrate using hardand soft bake techniques. The photoresist layer is then partially etchedback to expose the top of the ridge 216.

FIG. 4C shows the next step in which the insulation layers are etchedaway from the top of the ridge 216. This leaves the upper cladding layerexposed only in the region of the ridge.

As shown in FIG. 4D, the P-metal contact layer 222 is deposited.Typically, this contact layer is made from a gold alloy. Thereafter, thegold bond pads 226 are formed.

FIGS. 5A and 5B show an alternative ridge designs, to which the presentinvention is also applicable, in which similar reference numerals beingused to illustrate similarities in construction. Here, the ridge is notdefined by trenches as illustrated in FIGS. 4A-4D, but the surroundingupper cladding layer is completely etched away.

Specifically, a lower cladding layer 212 and an upper cladding layer 214sandwich an active layer 210. On top of the upper cladding layer is aninsulation layer 314, which isolates the upper cladding layer 214 fromthe P-metal contact layer 222. The contact layer, however, is inelectrical contact with the upper cladding layer 314 in the region ofthe ridge 216. On one or both sides of the ridge 216, bond pads 226 areformed, which also provide for ridge protection.

In the alternative embodiment illustrated in FIG. 5A, the ridge width isas defined earlier, i.e., is 1-7 μm, preferably 5-6 μm. Width is definedslightly differently, since the sides of the ridge are sloped, ratherthan vertical as shown in FIGS. 4A-4D. Here, the ridge width is definedas the width of the ridge, three-quarters of the way down the slopingside wall 410 of the upper cladding layer 214.

In the reverse ridge embodiment illustrated in FIG. 5B, the ridge widthis also as defined earlier, i.e., is 1-7 μm, preferably 5-6 μm. Widthhere is the width at the base of the ridge, w.

In still other implementation, the ridge is buried/planarized bydepositing photoresist, polyimide, semiconductor material, or othermaterial on either side of the ridge.

As is known in the art, many tens of ridges are typically formedside-by-side and parallel to each other along a single substrate/wafer.After the fabrication steps have been completed, the wafer is typicallyscribed and cleaved along the planes that run perpendicular to theridges along the length of the wafer. This yields what are termed bars.Typically, the facet coatings for the reflective rear facet 216 andpartially or antireflecting front facet 116 are then applied to thesebars. Thereafter, the bars are scribed and cleaved between successiveridges to form individual semiconductor laser devices as shown in FIG.2.

Modification to the profiles are possible, and contemplated by thepresent invention. Specifically, FIGS. 6A-6B illustrate two changes tothe overall ridge profile.

Specifically, as shown in FIG. 6A, in one embodiment, the ridge 216 isalso flared in the direction of the front facet 116 to further avoidcatastrophic facet damage at that facet also.

FIG. 5B shows another ridge profile in which the ridge 216 is flared inthe direction of the rear facet 217, but then the flared terminated justbefore the rear facet and the width is held constant.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims.

What is claimed is:
 1. A ridge waveguide pump laser module that isadapted to generate light in the 1.2-1.6 μm wavelength range, the modulecomprising: a ridge waveguide laser chip comprising a back, reflectivefacet and front facet through which the laser chip emits light, whereina ridge width increases in a direction of the back facet; and a opticalfiber pigtail, an end of which is positioned to receive and transmitlight emitted from the front facet by the laser chip.
 2. A ridgewaveguide pump laser module as claimed in claim 1, wherein a medial tofront section ridge width is 1.5 to 7 μm.
 3. A ridge waveguide pumplaser module as claimed in claim 2, wherein a rearward section ridgewidth is 3-13 μm.
 4. A ridge waveguide pump laser module as claimed inclaim 2, wherein a rearward section ridge width is 8-9 μm.
 5. A ridgewaveguide pump laser module as claimed in claim 1, wherein medial tofront section ridge width is 5 to 6 μm.
 6. A ridge waveguide pump lasermodule as claimed in claim 5, wherein a rearward section ridge width is3-13 μm.
 7. A ridge waveguide pump laser module as claimed in claim 5,wherein a rearward section ridge width is 8-9 μm.
 8. A ridge waveguidepump laser module as claimed in claim 1, wherein a rearward sectionridge width is 3-13 μm.
 9. A ridge waveguide pump laser module asclaimed in claim 1, wherein a rearward section ridge width is 8-9 μm.10. A ridge waveguide pump laser module as claimed in claim 1, whereinthe end of the optical fiber pigtail, which is positioned to receivelight generated by the ridge waveguide laser chip, comprises a flat-topwedge shaped fiber lens.
 11. A ridge waveguide pump laser module asclaimed in claim 1, wherein a core of the optical fiber pigtail iselliptical at least at an end that is positioned to receive lightgenerated by the laser chip.
 12. A ridge waveguide pump laser module asclaimed in claim 11 wherein a width of the core is tapered in adirection of light propagation away from the laser chip.
 13. A ridgewaveguide pump laser module as claimed in claim 1, wherein a distancebetween an active layer and a base of the ridge of the laser chip is0.3-0.9 μm.
 14. A ridge waveguide pump laser module as claimed in claim1, wherein a distance between an active layer and a base of the ridge ofthe laser chip is 0.6-0.7 μm.
 15. A ridge waveguide pump laser module asclaimed in claim 1, wherein the module is used to pump an EDFAamplifier.
 16. A ridge waveguide pump laser module as claimed in claim1, wherein the module is used to pump a Raman amplifier.
 17. A ridgewaveguide pump laser module as claimed in claim 1, wherein the module isused to pump a dispersion compensated fiber.
 18. A ridge waveguide pumplaser module as claimed in claim 1, wherein the laser chip is composedof InGaAsP.
 19. A ridge waveguide pump laser module as claimed in claim1, wherein the laser chip is composed of InGaAlAs or InGaAlAs andInGaAsP.
 20. A ridge waveguide pump laser module as claimed in claim 1,wherein the optical fiber is a polarization maintaining (PM) fiber.